Digital rights management (DRM) encompasses schemes for protecting the copyrights of digital content that is typically distributed online. Such digital content may include multimedia content, such as audio and/or video, as well as digital versions of books, magazines, and periodicals. One common type of DRM scheme is to enable access to digital content for a limited-use period. After the period has expired, the digital content is no longer accessible.
Typically a user of a device, such as a general-purpose computer or a special-purpose device like a music playing device, downloads content from a server over a network, like the Internet. The content may have an expiration date, such that it is accessible by the user for a period of limited use. The device thus has to track time including the time when the content was downloaded, and the length of time that has elapsed since the content was downloaded, in order to determine if the limited-use period has expired.
The device of the user can be referred to as an untrusted time device, because the owner or manager of the content that is permitting the content to be downloaded into the device does not control the internal system clock of the device, and cannot guarantee that the device will maintain reliable timekeeping. For instance, given content may have an expiration date of seven days from the date it was downloaded. To improperly extend the period of time in which the user can access the content, the user may repeatedly set back the system clock of the device, so that it appears that the content has not expired. Because the owner or manager of the content cannot control the internal system clock of the device, the device is thus an untrusted time device.
To avoid such scenarios in which the system clock of an untrusted time device is manipulated to circumvent DRM for content, the prior art provides for a number of solutions that the owner of manager of the content may employ. First, the content may not be accessed by the user at the untrusted time device unless this device is communicatively connected to a trusted time device. A trusted time device can be a device that is controlled by the owner or manager of the content, such that it can be guaranteed that the device will maintain reliable timekeeping. The trusted time device may be communicatively connectable to the untrusted time device over a network, such as the Internet, or a wired or wireless telephony network, like a cellular phone network.
Requiring that the content be accessible by the user only when the untrusted time device is communicatively connected to the trusted time device is burdensome and can severely restrict honest usage of the content. For example, in the case of a portable music playing device, requiring that the device be communicatively connected to the trusted time device for access of the content means that the content cannot be played while the user is jogging, is on an airplane, and so on. This prior art solution is thus untenable.
Another prior art solution is to detect when the internal system clock of the untrusted time device has been rolled back to a previous time, and not permit access of the content at the device until connection has been reestablished with the trusted time server. However, not all manipulations of the internal system clock of the untrusted time device by the user are malicious. The user may have changed time zones, for instance, requiring him or her to change the system clock of the device, or the user may be correcting minor errors in the time being kept by the clock. Therefore, such legitimate clock manipulations causing the inaccessibility of content can be inconvenient and frustrating for the user, resulting in this prior art solution to be untenable as well.